This invention relates to an analog rate changer for altering the frequency spectrum of audio signals and, more particularly, to an electronic analog rate changer which effects the change in frequency spectrum by controlling storage media of the fall-through register type.
An analog rate changer is a device for altering the frequency spectrum of an analog signal. The most common use for an analog rate changer is the time compression and expansion of prerecorded speech. As discussed in Time Compression and Expansion of Speech by the Sampling method by Francis F. Lee in Journal of the Audio Engineering Society, 20(9), pp. 738-742, Nov. 1972, the advantages of using prerecorded speech for information dissemination are numerous. Generally, neither literacy nor sight is required to receive information by this medium. The major disadvantage is that if the prerecorded speech is played back at the same rate at which it is prerecorded, the rate at which the information is received is limited to the rate at which the original speech was produced, which is normally around 110-175 words per minute.
Fortunately, people can listen to and comprehend speech at rates considerably greater than the rates at which words are normally spoken. Thus, the rate at which information can be received by listening to playback of prerecorded speech can be increased simply by playing back the prerecorded speech at a rate faster than at which it was originally recorded. Unfortunately, as the speed of playback is increased relative to the speed of recording, all of the frequency components of the audio speech signals are proportionately increased, i.e., the pitch or frequency spectrum of the signal is increased, This increase in playback speed distorts the quality of the audio signal, and at a playback speed 1.7 or greater than recording speed, the increased pitch results in a substantial reduction in intelligibility. But for the increased pitch, intelligibility and comprehension could be maintained at much greater relative playback speed. When the playback speed is reduced relative to the recording speed, the frequency spectrum of the audio speech signal is proportionately reduced. The reduced pitch of the speech signal likewise results in deterioration of intelligibility and comprehension.
Various signal processing techniques are known which are designed to reduce the pitch of audio speech signals from a prerecorded medium which is played back at a rate faster than it was recorded and to increase the pitch of the audio speech signal when the prerecorded medium is played at a rate slower than at which the signals were recorded, so that comprehension may be maintained at greater levels of speech compression and expansion.
One known technique of electronic time compression and expansion of speech is discussed in the aforementioned article of Francis F. Lee. In this technique, a pair of analog shift registers are alternately used for input and output purposes with signals being clocked into and out of only one register at a time. When one of the registers is accepting input samples, the other is providing signal samples for output and vice versa. In a time compression mode, the input signals are shifted into the register at a rate faster than at which output signal samples are taken from the other register. A first segment of information which is shifted into the input register at the instant of input-output role switching is lost or discarded, and this discarded interval increases with increases in playback speed. In a time expansion mode, each segment of information is repeated in part through recirculation control of the shift register. Input sampling is at a lower rate than output sampling, and input-output role switching is done when the input register is filled to capacity.
Another technique of time compression and expansion of speech which is discussed in the aforementioned article of Francis F. Lee employs a random access memory. Input samples are located into the memory in succession at one rate and output samples are retrieved at another rate greater or lesser than the input rate, depending upon whether time expansion or time compression is desired.
Various techniques are shown in U.S. Pat. No. 3,786,195 of Schiffman, some of which employ a variable delay line, some of which utilize digital shift registers, and one of which utilizes a random access memory.
Apparatus for audio signal time compression and expansion employing a delay line is likewise shown in U.S. Pat. Nos. 3,594,513 and 3,480,737 of Greenberg et al.